Gzip

Gzip (GNU zip) is a compression utility.

File format

The gzip file (.gz) format consists of:

• a file header
• optional headers
  • extra fields
  • original file name
  • comment
  • header checksum
• compressed data (commonly used compression method DEFLATE, without zlib header)
• a file footer

Characteristics	Description
Byte order	little-endian
Date and time values	POSIX timestamp Number of seconds since January 1, 1970 00:00:00 UTC
Character strings	ISO 8859-1 (LATIN-1)

File header

The file header is 10 bytes in size and contains:

Offset	Size	Value	Description
0	2	0x1f 0x8b	Signature (or identification byte 1 and 2)
2	1		Compression Method
3	1		Flags
4	4		Last modification time Contains a POSIX timestamp.
8	1		Compression flags (or extra flags)
9	1		Operating system Value that indicates on which operating system the gzip file was created.

Compression method

Value	Identifier	Description
0 - 7		Reserved
8	deflate	deflate compressed data

Flags

Value	Identifier	Description
0x01	FTEXT	If set the uncompressed data needs to be treated as text instead of binary data. This flag hints end-of-line conversion for cross-platform text files but does not enforce it.
0x02	FHCRC	The file contains a header checksum (CRC-16)
0x04	FEXTRA	The file contains extra fields
0x08	FNAME	The file contains an original file name string
0x10	FCOMMENT	The file contains comment
0x20		Reserved
0x40		Reserved
0x80		Reserved

Notes:

• Reserved flags bits must be zero.
• The FHCRC bit was never set by versions of gzip up to 1.2.4, even though it was documented with a different meaning in gzip 1.2.4.

Compression flags

This value contains flags specific to the compression method.

Compression flags - deflate

If compression method value is 8 (deflate) the following compression flags can be used:

Value	Identifier	Description
0x02		compressor used maximum compression, slowest algorithm
0x04		compressor used fastest algorithm

Operating System

Value	Identifier	Description
0		FAT filesystem (MS-DOS, OS/2, NT/Win32)
1		Amiga
2		VMS (or OpenVMS)
3		Unix
4		VM/CMS
5		Atari TOS
6		HPFS filesystem (OS/2, NT)
7		Macintosh
8		Z-System
9		CP/M
10		TOPS-20
11		NTFS filesystem (NT)
12		QDOS
13		Acorn RISCOS
255		unknown

Optional headers

Extra fields

This value is present in the file if the FEXTRA flag is set in the file header flags.

The extra field are variable of size and contains:

Offset	Size	Value	Description
0	2		Extra field data size Value in bytes.
2	...		Extra field data

Original file name

This value is present in the file if the FNAME flag is set in the file header flags.

This is the original name of the file being compressed, with any directory components removed, and, if the file being compressed is on a file system with case insensitive names, forced to lower case.

Contains an ISO 8859-1 (LATIN-1) string with end-of-string character.

Comment

This value is present in the file if the FCOMMENT flag is set in the file header flags.

Contains an ISO 8859-1 (LATIN-1) string with end-of-string character. Line breaks should be denoted by a single line feed character.

Header checksum

The header checksum contain a CRC-16 that consists of the two least significant bytes of the CRC-32 for all bytes of the gzip header up to and not including the CRC-16.

File footer

The file footer is 8 bytes in size and contains:

Offset	Size	Value	Description
0	4		Checksum (CRC-32)
4	4		Uncompressed data size Value in bytes.

See Also

• bzip2
• tar

External Links

• Source
• The gzip file format, by the gzip project
• The gzip compression algorithm, by the gzip project
• RFC1952: GZIP file format specification, by IETF
• Wikipedia: gzip